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Abstract:

Low phosphorus availability induces the formation of cortical
aerenchyma in roots. The adaptive significance of this response is unknown. We
hypothesized that aerenchyma may be helpful to low-phosphorus plants by reducing
root respiratory and phosphorus requirements, thereby increasing the metabolic
efficiency of soil exploration. To test this hypothesis we investigated
aerenchyma formation, root respiration and tissue phosphorus concentration in
maize and common bean genotypes in response to phosphorus availability and
ethylene treatments. Genotypes differed substantially in their ability to form
aerenchyma in response to low phosphorus. Aerenchyma formation was
disproportionately correlated with reduced root respiration; roots with 30%
cross-sectional area as aerenchyma had 70% less respiration than roots without
aerenchyma. Aerenchyma formation was also proportionally correlated with reduced
root phosphorus concentration. Variation in aerenchyma formation was correlated
with root respiration and phosphorus concentration, regardless of whether such
variation was caused genetically or by ethylene or phosphorus treatments.
Results with isolated roots were confirmed by measurement of whole root
respiration of intact maize plants. Our results support the hypothesis that
aerenchyma formation reduces the respiratory and phosphorus requirements of soil
exploration by roots, and thus, represents a useful adaptation to low phosphorus
availability.